1. Field of the Invention
The present invention relates to a method of manufacturing a semiconductor device, and more particularly to a method of forming a gate electrode in a semiconductor device.
2. Description of the Related Art
In general, a gate electrode of a MOS transistor has been formed of a polysilicon layer. The polysilicon layer has the advantage of being stable in forming process. However, as high integration of semiconductor device, the line widths of a gate electrode and other patterns become fine. Recently, the line width is reduced below 0.15 .mu.m. Therefore, there is problem that it is difficult to apply the doped polysilicon layer to a gate electrode material in a high speed device, since the doped polysilicon layer has a high resistivity.
This problem is also growing more and more serious as the high integration of the semiconductor. Accordingly, to overcome the problem, a gate electrode with a polycide structure in which a silicide layer using refractory metal such as tungsten(W) and titanium(Ti) is formed on the doped polysilicon layer, is proposed. However, since the gate electrode with the polycide structure is limited to speed up an ultra high integration semiconductor device, recently a technique of using refractory metal such as tungsten(W) as a gate electrode material has been researched and developed.
FIG. 1 is a cross sectional view showing a W-gate electrode according to a conventional art.
Referring to FIG. 1, a gate oxide layer is grown on a silicon substrate 10 and a doped polysilicon layer 12 is deposited thereon. A WN layer 13 as a diffusion barrier and a W layer 14 are then deposited on the polysilicon layer 12, in sequence and a mask oxide layer 15 is formed thereon.
Next, the mask oxide layer 15, the W layer 14, the WN layer 13 and the polysilicon layer 12 are sequentially etched to form a gate electrode. Thereafter, for removing damage and recovering the reliability of the gate insulating layer 11 due to the etching process, a gate re-oxidation process is performed to form a thermal oxide layer 17 on the side wall of the gate electrode.
Here, the gate re-oxidation process is performed at high temperature under oxygen atmosphere. However, the W layer 14 is abnormally oxidized by the gate re-oxidation process, to form a tungsten oxide(WO.sub.3) layer 16 on the side wall of the W layer 14, thereby deteriorating the morphology of the gate electrode, as shown in FIG. 1. Therefore, it is difficult to perform a side wall spacer process and ion-implantation for forming source/drain subsequently, thereby deteriorating device properties. Furthermore, in case the W layer 14 is extremely oxidized, its W content decreases, thereby increasing the resistivity of the gate electrode.
To overcome the problems, N. Yamamoto proposes Wet Hydrogen Oxidation process (refer to Journal of Electrochemical Society, Vol. 133, pp. 401. (1986)) as a new gate re-oxidation process. However, in this process, since oxidation process is performed at high temperature (for example, about 1,000.degree. C.) for long time in order to form a thermal oxide layer of a selected thickness, very large thermal budget occurs in semiconductor devices in which a metal layer may be used as a gate electrode material.
Furthermore, to overcome the problems, in case the gate re-oxidation process is performed under nitrogen atmosphere, the properties of gate oxide layer is not good comparing with using oxygen atmosphere.